Submersible pumping systems are often deployed into wells to recover petroleum fluids from subterranean reservoirs. Typically, the submersible pumping system includes a number of components, including one or more fluid filled electric motors coupled to one or more high performance pumps located above the motor. When energized, the motor provides torque to the pump, which pushes wellbore fluids to the surface through production tubing. Each of the components in a submersible pumping system must be engineered to withstand the inhospitable downhole environment.
Components commonly referred to as “seal sections” protect the electric motors and are typically positioned between the motor and the pump. In this position, the seal section provides several functions, including transmitting torque between the motor and pump, restricting the flow of wellbore fluids into the motor, protecting the motor from axial thrust imparted by the pump, and accommodating the expansion and contraction of motor lubricant as the motor moves through thermal cycles during operation. Prior art seal sections typically include a “clean side” in fluid communication with the electric motor and a “contaminated side” in fluid communication with the wellbore. Bellows or bags have been used to separate the clean side of the seal section from the contaminated side.
Recently, manufacturers have employed polymer expansion bags within the seal section to accommodate the expansion and contraction of motor lubricants while isolating the lubricants from contaminants in the wellbore fluid. Although generally effective at lower temperatures, the currently available polymers become somewhat permeable at extremely elevated temperatures and allow the passage of moisture across the membrane. The moisture reduces the insulating properties of polyimide and other films used to electrically isolate components within the downhole pumping system. Although piston-based systems may provide an alternative to the use of polymer expansion bags, prior art piston-based seal assemblies are susceptible to failure from sand, scale or other particulates. Moreover, the sealing rings used in existing pistons may deform under differential pressures, apply unwanted pressure against the interior of the seal section housing and reduce the movement of the piston. There is, therefore, a need for improved designs that can be used to accommodate expansion of motor fluids in elevated temperature applications. It is to this and other needs that the presently described embodiments are directed.